The present invention is directed towards the optics of luminance-chrominance systems for projection displays.
Optical projection systems in which the image is generated by light modulated by one or more xe2x80x9clightvalvesxe2x80x9d are becoming increasingly common. Devices such as televisions, presentation projectors and computer monitors have utilized such lightvalve based projection systems. Typically, in a single lightvalve system, a color image is produced by projecting red (R), green (G) and blue (B) (collectively referred to as xe2x80x9cprimaryxe2x80x9d hereinafter) image fields in a time sequential manner with sufficient rapidity that flicker is not apparent. The overall frame rate desired for color images is typically 60 Hertz or greater. Thus, the corresponding interval between each color image field is {fraction (1/180)}th of a second or less.
Single lightvalve systems such as those used in LCD (Liquid Crystal Display) projection systems are relatively inexpensive and the resulting performance is satisfactory. However, an inherent drawback of time sequential lightvalve and other systems is an effect known as xe2x80x98color breakup artifactxe2x80x99 or xe2x80x98field sequential color artifactxe2x80x99. Color breakup artifact manifests itself to a viewer as a transient rainbow-like fringing effect when rapid eye movements of several degrees are made. The effect is an inherent property of the human visual system but sensitivity to the effect varies greatly from person to person. Moreover, the seriousness of the effect depends strongly on the nature of the image being viewed.
An important issue in evaluating projection system methods is the total number of pixels required, because the cost and complexity of the display normally increase as the number of pixels required increases. In describing various projection systems it is convenient to express the number of pixels as a multiple of the number, N, which is the required number of pixels in the field sequential projection system. In addition to the number of pixels, the final screen luminance should be high enough so that it matches or exceeds the luminance provided by other display systems under commercialization. The final screen luminance of the whites of a projection system, can be expressed as a function of L, the final screen luminance of a field sequential projection system using a single reflecting lightvalve array with a polarizing cube.
A number of methods which have been traditionally used include the use of a separate projection system for each of red, green and blue lamps, or a single lamp with dichroic mirrors providing three separate red, green and blue beams. Another set of methods uses a large mosaic filter with a reflecting lightvalve array and polarizing beamsplitter.
FIGS. 1-4 show some of these methods that may be employed in projection systems. Projection system 100 of FIG. 1 uses three lamps (e.g. projection cathode-ray tubes) 110 to achieve the projection of red, green, and blue images in register on a reflecting or on a translucent screen. This has achieved commercial success, but the cost is high. The light from lamps 110 are filtered by interference filters 120, three in number, which will each increase the luminance of the red, green and blue beams as compared to when using dye-based filters. The red, green and blue light beams resulting from the filtering are each passed thru condensers 130. Condensers 130 are a series of lensing elements that make the light beams spatially uniform (i.e. even out the power of light across the area of the beam). In projection system 100, three lightvalve arrays 140 are used, each within its own beam area, to project red, green, and blue images in register on a translucent screen 160. With the aid of projection lenses 150, the light from the arrays is focused and thus combined onto a single RGB point on screen 160. Registration problems can be reduced by using reflecting arrays since their size can be small compared to the size of the final image. The number of required pixels is 3N.
Alternatively, as shown in FIG. 2, the light from a single lamp is 210 split into red, green, and blue components by three dichroic mirrors 215. The light from dichroic mirrors 215 are filtered by interference filters 220, three in number, which will each increase the luminance of the red, green and blue beams as compared to when using dye-based filters. The red, green and blue light beams resulting from the filtering are each passed thru condensers 230 which make the beams of light spatially uniform. In projection system 200, three lightvalve arrays 240 are also used, each within its own beam area, to project red, green, and blue images in register on a translucent screen 260. With the aid of projection lenses 250, the light from the arrays are focused onto a single RGB lumen on screen 160.
Both projection systems 100 and 200 suffer the cost disadvantages of having three lightvalve arrays and three lenses, of maintaining the registration of the three projected images. The number of total pixels for projection system 200 is again 3N as with the three lamp system 100 of FIG. 1. Compared with the prism assembly method (shown below in FIG. 4), potential advantages are that, if three lamps are used as in system 100, the light output is tripled; or if one lamp is used with beam-splitting dichroic mirrors as in system 200, they are less costly to assemble and manufacture than the prism assembly methods.
An alternative approach is to abandon time sequential imaging while still using only one lightvalve by presenting the primary colors to the viewer in the space domain, rather than in the time domain. One way of constructing such a field sequential system would be to arrange the R, G and B pixels in a mosaic pattern, like the arrangement of phosphor spots in a Cathode Ray Tube device. The lightvalve would be illuminated using white light, and each R pixel would be covered with a red filter, each G pixel with a green filter and each B pixel with a blue filter. The requisite filter array would contain about 106 or more filters. Furthermore, in the case of a micro-display lightvalve array, each filter would measure only 10xc3x9710 xcexcm2. Though conceptually easy, implementing such large filter arrays and such small individual filters could be prohibitively expensive. Disadvantageously, mosaic filter arrays need about three times as many pixels.
FIG. 3 shows one such mosaic filter 300 that can be employed in a projection system. Mosaic filter 300 contains a pattern of red, green and blue pixel locations. In light-sensitive arrays (such as CCD arrays) more green cells are usually incorporated than red or blue cells. Where required mosaic filter 300 may instead be composed of cyan, magenta, yellow, and green cells or any combination of colors. For most displays, the relative numbers of red, green, and blue cells have to be chosen to maximize the luminance of the display and to keep the total number of cells to a minimum. To meet these requirements an equal number of red, green, and blue cells are used as in the case of mosaic filter 300.
Single transmitting LCD arrays, covered with a mosaic of red, green, and blue filters, such as filter mosaic 300, provide either panel displays illuminated by fluorescent lamps or projected displays by means of overhead projectors. In projection system 400 of FIG. 4, a reflecting LCD array 430 is used with mosaic filters 440 to provide a projected image based on light originating from a single lamp 410. Reflecting array 440 is used because its small size makes a compact projector possible. Condenser 420 makes the light from lamp 410 to be spatially uniform. Field lens 450 transmits the image onto the appropriate position on projection lens 490. The image rays are first passed through a polarizing cube 470 which allows any light arriving in a given direction X to pass through its hypotenuse while reflecting back any light perpendicular to that direction X. The projection lens 490 focuses the filtered light output from polarizing cube 470 onto a screen or other destination (not shown). Some advantages of projection system 400 are that only one light-valve array is used, no temporal color break-up can occur, and no prism assembly is required. A disadvantage is that the filters 440 have to be made of dyes and thus have lower transmittances than the interference filters used in the filter wheel of the field sequential system or in the dichroic elements of the prism assembly. The number of pixels required for this mosaic method is usually 3N.
One other arrangement of avoiding the mosaic filters is to provide a rapidly revolving red, green, and blue filter wheel in front of a single reflecting LCD array with a polarizing beam-splitting cube. The use of a reflecting array makes it possible to incorporate micro-circuits beneath the pixels without loss of light; and illuminating them with a polarizing beam-splitter cube makes for a convenient and efficient arrangement. However, the field sequential system provided by the rotating filter wheel results in disturbing color break-up for some observers. The use of three lightvalve arrays, with a dichroic prism assembly to combine the three images, avoids the color break-up, but involves the added costs of two extra arrays and the prism assembly.
Alternative methods of projecting images, using lightvalve arrays to produce color displays, are therefore of interest.
The invention in one or more embodiments consists of a lenticular array and one or more lightvalves together with lens filters that serve to synthesize mosaic filters in an optical projection system that produces an image.
In various embodiments, the filters correspond and align with the shape and configuration of lenticules in the lenticular array. Also, in various embodiments, the lightvalves may be of a transmitting or reflecting type and in most embodiments, the lenticular array is built onto one of the lightvalves. Further, in various embodiments, the luminance and chrominance components of the image to be projected may be processed into separate signals.